In an impact modifier, the impact resistance comes from the constituent crosslinked, low glass transition (Tg) rubber. The characteristics of this rubber directly affect the ability of the modifier to provide ductility in the host polymer during impact, and the most important of these characteristics is crosslink density, as measured by the swell ratio, because it controls the stiffness of the rubber and its ability to cavitate. Often, in an impact modifier, the crosslink density is controlled by the addition of a crosslinker into the low Tg polymer. The distribution of the crosslink density and, thus, the swell ratio are directly proportional to the distribution of the crosslinker in the rubber. The rubber is also often made with a shot process, whereby all the monomer is charged to the reactor and then its polymerization is initiated, in order to keep the standing monomer concentration high and the branching in the rubber low; therefore, the crosslinker distribution and the resulting crosslink density distribution are entirely dependent on the reactivity ratios of the rubber monomer and crosslinker. There is a need, therefore, for an alternative to the shot emulsion polymerization process that will provide a more uniform crosslink density distribution.